The invention will be described with reference to providing protective and decorative finishes on exterior automotive panels, but it should be understood that the automobile is only one of many substrates to which the sheet material of the invention can be applied.
As pointed out in an article by Alan J. Backhouse entitled "Routes To Low Pollution Glamour Metallic Automotive Finishes", Journal of Coatings Technology, Vol. 54, No. 693, pages 83-90, October 1982, there is a growing need to reduce the amount of atmospheric pollution caused by solvents emitted during industrial painting processes. Many different approaches have been proposed. For example, efforts have been made to replace the solvent-based paints used for automobiles with water-based paints. Work has also been done on the use of high solids formulations to lessen the emission or organic solvents. However, the application of automotive finishes is a highly demanding art because of the extremely high quality of the surface finish required and because of the common application of metallic finishes to provide what Backhouse refers to as "high stylistic effects". Accordingly, past efforts to replace the low viscosity, low-solids-content paint formulations conventionally used in spray painting operations in the automotive industry have met with limited success.
A more promising approach is to eliminate entirely the need for spray painting. Elimination of spray painting, or reduction in its use, would not only reduce atmospheric pollution, but would provide cost savings in that spray painting operations are so wasteful that more than half of the paint may be wasted. A means for achieving such goal exists through the use of a pre-formed thermoplastic sheet material which can be bonded to the panel to provide the protective and decorative coating. Such techniques have been utilized for interior automobile panels as described, for example, in U.S. Pat. No. 3,551,232 issued Dec. 29, 1970.
The objective of U.S. Pat. No. 3,551,232 is to overcome the problems of bubbling and blistering of the resin sheet that tend to occur in the vacuum-forming process. It achieves this by use of an adhesive containing an inert particulate filler which minimizes the entrapment of air.
To employ a process of the type described in U.S. Pat. No. 3,511,232 with exterior automotive panels presents a greater challenge. The surface appearance of such panels is of critical importance, so that it is necessary not only to avoid bubbling or blistering caused by entrapped air, but to provide a protective and decorative coating that will equal or exceed the quality of a spray-painted surface. Furthermore, exterior automotive panels present a particular problem in view of the difficulty of smoothly adhering a flexible sheet material to a curved substrate and the difficulty of doing so while maintaining over the entire surface a uniform color intensity.
Efforts have been made by others to produce a flexible and stretchable sheet material having these capabilities through the application of one or more paint layers to the surface of a thermoformable polymeric support. However, prior to the invention of Reafler, U.S. patent application Ser. No. 116,426 now abandoned, it is believed that these efforts have resulted in products having numerous coating defects and in which the paint layer does not have the high degree of uniformity which permits the sheet material to undergo the stresses of thermoforming and yet meet the exacting standards of an exterior automotive finish.
In accordance with the invention of the Reafler patent application, a flexible and stretchable sheet material that is (1) capable of meeting the requirements of an exterior automotive finish and (2) capable of withstanding the stretching and bending forces involved in bonding it to exterior automotive panels is produced by precision coating techniques which provide exacting control of the thickness and thickness uniformity of the coatings. Moreover, such coating techniques provide essentially defect-free coatings having a substantially uniform quality and appearance.
The sheet material of the Reafler application comprises a thin flexible carrier film, and a protective and decorative paint layer, also known as a basecoat, adhered to one surface of the carrier film. The carrier film has heat-softening and tensile elongation properties which adapt it to use in the thermoforming process and the paint layer has compatible heat softening and tensile elongation properties. As a result, the sheet material can undergo substantial elongation without crazing or delamination of the protective and decorative paint layer.
The sheet material has a substantially unstressed relaxed state and a relaxed area and is heat softenable to a substantially plastic state in which it is plastically extendable and formable over irregular surfaces to an extended state having an extended area at least 50% greater than the relaxed area. The protective and decorative paint layer has a substantially uniform quality and appearance both in the relaxed and extended states. The sheet material can be stretched and bonded to a three-dimensional substrate as a smooth and wrinkle-free protective and decorative coating of uniformly attractive appearance.
The Reafler application also discloses a process for the manufacture of the sheet material comprising the steps of: providing a thin, flexible carrier film; providing a fluid protective and decorative film-forming composition; forming a laminar flow of the composition and directing the flow into contact with the surface of the carrier film to form thereon a protective and decorative layer of substantially uniform thickness; and converting the protective and decorative layer to a dry and tack-free state. The protective and decorative film-forming composition is advantageously coated over an adhesion-promoting tie-layer. The process provides an essentially defect-free coating with a high degree of uniformity of layer thickness and a substantially uniform quality and appearance.
In an important embodiment of the sheet material, for which the present invention provides an improvement, the paint layer comprises a film-forming binder, a colorant and generally flat, light-reflecting particles which are oriented by the laminar flow process; that is, they are preferentially oriented substantially parallel to the surface of the layer. This provides a paint layer characterized by a high degree of geometric metamerism, a property commonly referred to as "flop."
The preferred embodiment of the sheet material, for which the present invention provides an improvement, also includes a transparent topcoat comprising a clear or lightly colored polymer. This topcoat is also called a clearcoat. The carrier film, paint layer and topcoat layer have compatible heat-softening and tensile elongation properties which render the sheet material suitable for use in a thermoforming process. Also the carrier film, paint layer and topcoat layers are of such uniform character that the sheet material has substantially the same uniformity of appearance before and after the thermoforming process.
The transparent topcoat provides a number of desired properties. These include resistance to abrasion and to weathering. It can also improve the surface qualities of gloss and distinctness of image (DOI). Since the sheet materials are especially intended for use as automotive coverings which demand high gloss and other surface qualities; further improvement in these qualities is desirable. Especially it is desirable to reduce or eliminate any tendency toward loss of gloss or of DOI when the material is stretched during thermoforming. Since the paint layer and clearcoat are coated on stretchable carrier films rather than on rigid surfaces and are coated by laminar flow methods rather than by the usual spraying processes for automotive finishes, the existing automotive finishing literature gives little or no guidance as to ways for improving or retaining the gloss or DOI of these sheet materials when stretched.